In this study, the chemically polymerized PPy and PANi films with different selectivity by controlling dedoping time were fabricated. Additionally, the sensing properties and mechanism of VOCs adsorption to conducting polymers were investigated with contact angles measurement, a scanning probe microscope (SPM) and a UV-Vis-NIR spectrophotometer. The thin sensor had higher sensitivity compared to the thick one, which the dedoped sensor at I-minute had the highest sensitivity. Upon gas absorption, polypyrrole exhibited positive sensitivity while polyaniline had negative sensitivity. PPy film showed hydrophilic property and PANi film showed hydrophobic property. After gas absorption, the sensitivity increased as a function of the polarity of the absorbed molecules. These behaviors are due to the polar molecules absorbed with the movable polaron or free carrier, which then interrupts or generates the movement of polaron and carrier, and then changes the conductivity of the polymer. We found that conducting polymer sensors are very sensitive to the difference in polarity of gas molecules.